# Modify string by inserting characters such that every K-length substring consists of unique characters only

Given string S of size N consisting of K distinct characters and (N – K) ‘?’s, the task is to replace all ‘?’ with existing characters from the string such that every substring of size K has consisted of unique characters only. If it is not possible to do so, then print “-1”.

Examples:

Input: S = “????abcd”, K = 4
Output: abcdabcd
Explanation:
Replacing the 4 ‘?’s with “abcd” modifies string S to “abcdabcd”, which satisfies the given condition.

Input: S = “?a?b?c”, K = 3
Output: bacbac
Explanation:
Replacing S[0] with ‘b’, S[2] with ‘c’ and S[4] with ‘a’ modifies string S to “bacbac”, which satisfies the given condition.

Approach: The idea is based on the observation that in the final resultant string, each character must appear after exactly K places, like the (K + 1)th character must be the same as 1st, (K + 2)th character must be the same as 2nd, and so on.

Follow the steps below to solve the problem:

Below is the implementation of the above approach:

## C++

 `// C++ program for the above approach` `#include ` `using` `namespace` `std;`   `// Function to replace all '?'` `// characters in a string such` `// that the given conditions are satisfied` `void` `fillString(string s, ``int` `k)` `{` `    ``unordered_map<``int``, ``char``> mp;`   `    ``// Traverse the string to Map the` `    ``// characters with respective positions` `    ``for` `(``int` `i = 0; i < s.size(); i++) {` `        ``if` `(s[i] != ``'?'``) {` `            ``mp[i % k] = s[i];` `        ``}` `    ``}`   `    ``// Traverse the string again and` `    ``// replace all unknown characters` `    ``for` `(``int` `i = 0; i < s.size(); i++) {`   `        ``// If i % k is not found in` `        ``// the Map M, then return -1` `        ``if` `(mp.find(i % k) == mp.end()) {`   `            ``cout << -1;` `            ``return``;` `        ``}`   `        ``// Update S[i]` `        ``s[i] = mp[i % k];` `    ``}`   `    ``// Print the string S` `    ``cout << s;` `}`   `// Driver Code` `int` `main()` `{` `    ``string S = ``"????abcd"``;` `    ``int` `K = 4;` `    ``fillString(S, K);`   `    ``return` `0;` `}`

## Java

 `// Java Program to implement` `// the above approach` `import` `java.io.*;` `import` `java.util.*;`   `class` `GFG {`   `    ``// Function to replace all '?'` `    ``// characters in a string such` `    ``// that the given conditions are satisfied` `    ``static` `void` `fillString(String str, ``int` `k)` `    ``{`   `        ``char` `s[] = str.toCharArray();`   `        ``HashMap mp = ``new` `HashMap<>();`   `        ``// Traverse the string to Map the` `        ``// characters with respective positions` `        ``for` `(``int` `i = ``0``; i < s.length; i++) {` `            ``if` `(s[i] != ``'?'``) {` `                ``mp.put(i % k, s[i]);` `            ``}` `        ``}`   `        ``// Traverse the string again and` `        ``// replace all unknown characters` `        ``for` `(``int` `i = ``0``; i < s.length; i++) {`   `            ``// If i % k is not found in` `            ``// the Map M, then return -1` `            ``if` `(!mp.containsKey(i % k)) {` `                ``System.out.println(-``1``);` `                ``return``;` `            ``}`   `            ``// Update S[i]` `            ``s[i] = mp.getOrDefault(i % k, s[i]);` `        ``}`   `        ``// Print the string S` `        ``System.out.println(``new` `String(s));` `    ``}`   `    ``// Driver Code` `    ``public` `static` `void` `main(String[] args)` `    ``{`   `        ``String S = ``"????abcd"``;` `        ``int` `K = ``4``;` `        ``fillString(S, K);` `    ``}` `}`   `// This code is contributed by Kingash.`

## Python3

 `# Python 3 program for the above approach`   `# Function to replace all '?'` `# characters in a string such` `# that the given conditions are satisfied` `def` `fillString(s, k):` `    ``mp ``=` `{}`   `    ``# Traverse the string to Map the` `    ``# characters with respective positions` `    ``for` `i ``in` `range``(``len``(s)):` `        ``if` `(s[i] !``=` `'?'``):` `            ``mp[i ``%` `k] ``=` `s[i]`   `    ``# Traverse the string again and` `    ``# replace all unknown characters` `    ``s ``=` `list``(s)` `    ``for` `i ``in` `range``(``len``(s)):` `      `  `        ``# If i % k is not found in` `        ``# the Map M, then return -1` `        ``if` `((i ``%` `k) ``not` `in` `mp):` `            ``print``(``-``1``)` `            ``return`   `        ``# Update S[i]` `        ``s[i] ``=` `mp[i ``%` `k]`   `    ``# Print the string S` `    ``s ``=`   `''.join(s)` `    ``print``(s)`   `# Driver Code` `if` `__name__ ``=``=` `'__main__'``:` `    ``S ``=` `"????abcd"` `    ``K ``=` `4` `    ``fillString(S, K)`   `    ``# This code is contributed by bgangwar59.`

## C#

 `// C# program for the above approach ` `using` `System;` `using` `System.Collections.Generic; `   `class` `GFG ` `{`   `  ``// Function to replace all '?'` `  ``// characters in a string such` `  ``// that the given conditions are satisfied` `  ``static` `void` `fillString(``string` `str, ``int` `k)` `  ``{`   `    ``char``[] s = str.ToCharArray();`   `    ``Dictionary<``int``, ` `    ``int``> mp = ``new` `Dictionary<``int``, ` `    ``int``>();`   `    ``// Traverse the string to Map the` `    ``// characters with respective positions` `    ``for` `(``int` `i = 0; i < s.Length; i++) {` `      ``if` `(s[i] != ``'?'``) {` `        ``mp[i % k] = s[i];` `      ``}` `    ``}`   `    ``// Traverse the string again and` `    ``// replace all unknown characters` `    ``for` `(``int` `i = 0; i < s.Length; i++) {`   `      ``// If i % k is not found in` `      ``// the Map M, then return -1` `      ``if` `(!mp.ContainsKey(i % k)) {` `        ``Console.WriteLine(-1);` `        ``return``;` `      ``}`   `      ``// Update S[i]` `      ``s[i] = (``char``)mp[i % k];`   `    ``}`   `    ``// Print the string S` `    ``Console.WriteLine(``new` `string``(s));` `  ``}`   `  ``// Driver code` `  ``static` `void` `Main()` `  ``{` `    ``string` `S = ``"????abcd"``;` `    ``int` `K = 4;` `    ``fillString(S, K);` `  ``}` `}`   `// This code is contributed by susmitakundugoaldanga.`

## Javascript

 ``

Output:

`abcdabcd`

Time Complexity: O(N)
Auxiliary Space: O(N)

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